GALILEO AND HIS JUDGES. 157 



into which we have separated the doctrine [these were, 1st, that 

 the force attracting different planets to the sun, and, 2nd, the force 

 attracting the same planet in different parts of its orbit, is as the 

 inverse square of the distances ; 3rd, that the earth exerts such a 

 force on the moon, and that this is identical with terrestrial gravity ; 

 4th, that there is a mutual attraction of the heavenly bodies on one 

 another ; 5th, that there exists a mutual attraction of all particles 

 of matter throughout the universe] would of itself have been 

 considered as an important advance, would have conferred distinc- 

 tion on the persons who made it, and the time to which it belonged. 

 All the five steps made at once formed not a leap, but a flight ; 

 not an improvement merely, but a metamorphosis ; not an epoch, 

 but a termination. Astronomy passed at once from its boyhood to 

 mature manhood. Again, with regard to the extent of the truth, 

 we obtain as wide a generalisation as our physical knowledge 

 admits when we learn that every particle of matter, in all times, 

 places, and circumstances, attracts every other particle in the 

 universe by one common law of action. And by saying that the 

 truth was of a fundamental and satisfactory nature, I mean that it 

 assigned, not a rule merely, but a cause, for the heavenly motions ; 

 and that kind of cause which most eminently and peculiarly we 

 distinctly and thoroughly conceive, namely, mechanical force. 

 Kepler's laws were merely formal rules, governing the celestial 

 motions according to the relations of space, time, and number; 

 Newton's was a causal law, referring these motions to mechanical 

 reasons. It is no doubt conceivable that future discoveries may 

 both extend and further explain Newton's doctrines; may make 

 gravitation a case of some wider law, and may disclose something 

 of the way in which it operates questions with which Newton 

 himself struggled. But, in the meantime, few persons will dis- 

 pute that, both in generality and profundity, both in width and 

 depth, Newton's theory is without a rival or neighbour.* 



The effect of all this on the Copernican system 

 and the evidence on which it rested, was to raise that 



* I do not think the truth of this is affected by any of the 

 great modern discoveries ; though that of the Conservation of 

 Energy approaches more nearly than others to Universal Gravi- 

 tation in its importance. 



